Disk drives using various kinds of disks, such as optical disks, magneto-optical disks, flexible magnetic-recording disks, and similar disks for data-storage are known in the art. In particular, hard-disk drives (HDDs) have been widely used as data-storage devices that have proven to be indispensable for contemporary computer systems. Moreover, HDDs have found widespread application to motion picture recording and reproducing apparatuses, car navigation systems, cellular phones, removable memories used in digital cameras and similar devices, in addition to computers, because of their outstanding information-storage characteristics.
Magnetic-recording disks used in a HDD have multiple concentric data tracks and multiple servo sectors provided discretely in the circumferential direction. User data is recorded in units of data sectors; and, data sectors are recorded between servo sectors. A rotary actuator moves a head-slider above a spinning magnetic-recording disk. A magnetic-recording head of the head-slider accesses a desired data sector in accordance with position data indicated by a servo sector to write data to, or alternatively, to read data from, the data sector.
Servo sectors are recorded on magnetic-recording disks during HDD manufacturing. Typically, after mounting a magnetic-recording disk on a HDD, servo sectors are recorded on the magnetic-recording disk by mechanical control of a magnetic-recording head and an actuator to which the magnetic-recording head is affixed through electrical control of a voice-coil motor (VCM) attached to the actuator. The recorded servo data may deviate from ideal annular tracks, which occurs during servo data writing, or after servo data writing, because of eccentricity of the magnetic-recording disk, or other factors such as external vibration. Hence, servo data which the magnetic-recording head reads out from a servo sector includes a component referred to by the term of art, “repeatable run-out” (RRO), which is a measure of deviation from ideal annular tracks. Engineers and scientists engaged in the development of magnetic-recording technology are interested in further developing servo control systems to control the effects of RRO that may affect the high levels of reliability that have come to be expected by consumers in the market for HDDs.